Ultrasonic imagining catheters

ABSTRACT

An intravascular ultrasonic imaging catheter is provided with a flexible circuit electrically coupled to a transducer array mounted on the distal end of the catheter, a portion of the flexible circuit being helically wound about the catheter in order to enhance the flexibility of the circuit. The catheter may be a balloon catheter which is also provided with a stent mounted on the balloon, the stent carrying one or more drugs designed to be eluted or washed into a patient&#39;s blood stream after the stent ahs been delivered, by a the balloon catheter, into a target area within the patient&#39;s vascular system.

[0001] This invention relates to ultrasonic imaging catheters includingcatheters which are combined with a balloon device.

[0002] Catheters having an ultrasonic transducer array mounted on thedistal end of the catheter are known. Examples are the arrangementdisclosed in the present applicant's United Kingdom patent numbers: 2208 138; 2 212 267; 2 233 094 and 2 246 632.

[0003] Catheters fitted with a balloon which can be distended in orderto press plaque in a patient's artery back into the wall of the artery(a procedure known as angioplasty) in order to increase the crosssection of the artery's lumen are also well known.

[0004] In the applicant's published United Kingdom patent No. 2 325 020there is disclosed a catheter which has the combination of an ultrasonicarray at or near it's distal end combined with a balloon arrangementalso carried at or near the distal end of the catheter. With thearrangement disclosed in the applicant's United Kingdom Patent No. 2 287375 the ultrasonic imaging system includes a multiplexing arrangementwhich is also mounted on the catheter at or near it's distal end. Thisspecification also discloses a method of manufacturing thetransducer/multiplexer arrangement which method involves firstmanufacturing the arrangement in a flat configuration and then wrappingit into a cylindrical circular cross-section configuration. Thetransducer array and multiplexer arrangement are linearly separated fromone another but electrically inter-connected by what will be referred toas a flex circuit. The transducer array is cylindrical in its finalconfiguration as is the multiplexer arrangement, the latter consistingtypically of four multiplexer units, arranged in a cylindrical boxsection.

[0005] The catheters to which the present invention relates aretypically around one millimetre in external diameter. In the case wherethe catheter also incorporates an angioplast balloon and this balloon,when inflated, might typically have an external diameter of threemillimetres.

[0006] There is a requirement to be able to provide the ultrasonicimaging arrangement previously described either alone or in combinationwith a balloon arrangement previously described in a catheter having asmaller outside diameter. More specifically, in this art the diameter ofcatheters is expressed in terms of units known as “french” and thepresent invention is concerned with providing a combined ultrasonicimaging system/balloon catheter arrangement suitable for use with a2.4-2.9 french combination catheter. (1 French equals 0.013 inches)

[0007] There is a requirement that a stent can be pre-mounted onto theballoon and firmly attached so it does not detach.

[0008] There is a requirement that the combination catheter shouldsmoothly negotiate a six french guide catheter and a blood vessel with aseven millimetre bend radius and it should do so without damage to theelectrical circuitry and components associated with the transducer.

[0009] The following requirements/constraints apply when designing acatheter having an ultrasonic transducer array, an associatedmultiplexer arrangement, a balloon and a stent inserted on the balloon:

[0010] a) the balloon needs to be close to the ultrasonic array at thecatheter's distal end from a visualisation point of view;

[0011] b) the multiplexer needs to be close to the ultrasonic array inorder to minimise the length of the electrical connections between themand to enhance the rigidity of the distal tip of the catheter;

[0012] c) locating the mulitplexer within the balloon to meet therequirement of b) means that it needs to be electrically insulated fromthe saline solution used to inflate the balloon;

[0013] d) the stent needs to be securely mounted on the outside of theballoon so that it cannot be dislodged and can be radially expanded byinflation of the balloon;

[0014] e) one way to secure the stent on the outside of the balloon isto crimp it in position;

[0015] f) crimping the stent around the balloon can cause damage to theballoon and anything contained within it.

[0016] The present invention is concerned with designing the catheter insuch a way that these conflicting design requirements are substantiallymet.

[0017] According to a first aspect of the present invention a catheterballoon specified in the first aspect above is characterised in that themultiplexer units are longitudinally separated from the transducer, anda balloon device is positioned between the most distal multiplexer andthe transducer.

[0018] According to a second aspect of the present invention a catheterhaving an ultrasonic transducer including a plurality of multiplexerunits is characterised in that the multiplexer units are longitudinallyseparated from one another along the length of the catheter. Thecatheter may also include a balloon device.

[0019] According to a third aspect of the present invention a catheterhas an ultrasonic transducer arrangement located at or near its distalend, which arrangement includes a flexible circuit through whichelectrical power can be supplied to the transducer array, characterisedin that the flexible circuit incorporates at least one helically woundsection so constructed as to enhance the flexibility of that circuit andits ability to accommodate bending as the catheter is moved along anon-linear path such as an artery of a patient. The catheter may alsoinclude a balloon device.

[0020] According to a fourth aspect of the present invention thecatheter arrangement specified in the first aspect above ischaracterised in that there are flex circuits between adjacentlongitudinally separated multiplexer units, those flex circuits beingcharacterised in that they are helically wound in order to enhance theirflexibility and the overall catheter assembly's capability ofnegotiating bends in a non-linear path without damage being caused as aresult of the bending.

[0021] According to a fifth aspect of the present invention a catheterwhich includes an inflatable balloon device, an ultrasonic transducerarray and a flexible electrical circuit adapted to supply current to thetransducer array is characterised in that at least a portion of theflexible circuit is located within the balloon device and, in use, isdirectly exposed to the inflating fluid introduced into the balloondevice, i.e. that portion of circuit is not enclosed in a protectivetube but is insulated to function when immersed.

[0022] According to a sixth aspect of the present invention a stent iscrimped over the balloon, such that the crimping force does not damagethe multiplexer and transducer, and the helical flex circuit limits theminimum diameter the stent is crimped to.

[0023] How the invention may be carried out will now be described by wayof example only and with reference to the accompanying drawings inwhich:

[0024]FIG. 1 is a schematic representation of a combination catheterincorporating the present invention;

[0025]FIG. 2 is a fragmentary view on a larger scale showing a balloondevice, ultrasonic transducer array and associated multiplexerarrangement forming part of the overall arrangement shown in FIG. 1;

[0026]FIG. 3 is a side elevational fragmentary view on an enlarged scaleillustrating a guide wire exit to a catheter having an inner and outerbody at its distal end;

[0027]FIG. 4 comprises five cross-sectional views, FIGS. 4A, 4B, 4C, 4Dand 4E taken on the lines A-A, B-B, C-C, D-D and E-E respectively ofFIGS. 1 and 3.

[0028]FIG. 5 is a sectional elevational view on an enlarged scaleshowing that portion of the catheter which comprises the transducerarray adjacent the distal end of the balloon device, both balloon andtransducer being located at or near the distal end of the catheteritself;

[0029]FIG. 6 comprises FIG. 6A which shows that part of the catheterwhich includes the transducer array, flex circuit and multiplexers whenthey are in the flat condition and FIG. 6B which shows the samearrangement when they are in the wrapped condition; and

[0030]FIG. 7 is an enlarged prospective view of a drug loaded stentaccording to the present invention.

FIG. 1

[0031] This is a diagrammatic illustration of the overall catheterarrangement to which the present invention is applied.

[0032] The detailed construction is shown in the other figures of thedrawings in which the same reference numerals have been used todesignate the same or equivalent elements.

[0033] In FIG. 1 a catheter generally indicated at 1 has at its distalend A the combination of a balloon unit 2 and an ultrasonic annulartransducer array 3, the extreme distal end of the catheter comprising asoft tip 4.

[0034] Associated with the transducer array 3 is a multiplexingarrangement indicated at 5.

[0035] At the proximal end B of the catheter there is provided a ballooninflation port 6 by which fluid (typically a saline solution 19) can beintroduced through the catheter into the balloon 2 in order to inflateit in known manner.

[0036] Also at the proximal end B there is provided a connector 7 andassociated strain relief device 8 by which the proximal end B of thecatheter may be connected to a catheter interface module (CIM) and thusto the electronic imaging system.

[0037] At an intermediate point along its length the catheter isprovided with opaque markers 9 to assist the clinician in being able tosee the catheter within the patient's artery using x-ray equipment.

[0038] The ultrasonic transducer array 3 is provided with electricalpower by means of a ribbon cable 22 which runs the length of thecatheter, the proximal end of the ribbon cable being connected to anelectrical supply and control arrangement (not shown) which itself isnot part of the present invention.

[0039] The multiplexing arrangement 5 consists of a number ofmultiplexing units, in this case four, the functional purpose of whichis to reduce the number of electrical leads which would otherwise haveto be provided along the length of the catheter in order to energise thelarge number of transducer elements in the transducer array 3. Typicallythe number of elements would be sixty four. By having a multiplexingarrangement the number of electrical leads can be significantly reduced.The provision of such a multiplexing arrangement is known.

[0040] The present invention is concerned with the configuration andconstruction of the multiplexing arrangement and the associated flexiblecircuit and the relationship of the balloon device to the flexiblecircuit.

[0041] According to the present invention the individual multiplexingunits 5 a, 5 b, 5 c and 5 d are longitudinally spaced from one anotheralong the length of the catheter as indicated in FIG. 1, thelongitudinal spacing being identified as “S”.

[0042] In the arrangement of FIG. 1 the longitudinally spacedmultiplexing units 5 are located immediately proximal to the balloondevice 2 which in turn is proximal to the ultrasonic transducer array 3.

[0043] Mounted on the balloon is an optional stent 102 which is crimpedonto the balloon. The helical flex circuit 12 acts as a limit to thecrimping of the stent. The crimping operation involves application of aforce which would otherwise damage components inside, but in this designthe components are outside the stent region.

[0044] The construction of this distal end of the catheter isillustrated in more detail in FIGS. 2 to 5.

FIGS. 2 TO 5

[0045] The ultrasonic transducer array 3 consists of sixty-fourindividual transducer elements arranged in a cylindrical configuration,these elements being contained between a proximal ring 24 and a distalring 25 (FIG. 2). The multiplexer units 5 a, 5 b, 5 c and 5 d areelectrically connected to the transducer array 3 by means of a flexiblecircuit indicated at 12.

[0046] This flexible circuit 12 is arranged in a helical configurationand it passes from the transducer array 3 to the multiplexer units 5through the balloon device 2.

[0047] The balloon device 2 comprises a flexible and expandable balloonenvelope 13 which is sealed at 14 and 15 to the catheter.

[0048] That portion of the flexible circuit 12 which passes through theinside of the balloon unit 2 is insulated with a water proof coatingsuch as Parylene™ (Speciality Coatings Ltd, Northampton) and exposed tothe fluid (typically a saline solution 19) which is used to inflate theballoon envelope 13, i.e. that part of the flexible circuit 12 which iswithin the balloon envelope 13 is not contained within any protectivetube.

[0049] At its distal end the catheter 1 consist of an inner body or tube16 and an outer body or tube 17.

[0050] The catheter is inserted into a patient's artery after a metalguide wire 18 (see FIG. 3) has first been inserted into the artery. Thecatheter then in effect runs down the guide wire to bring the distal endof the catheter into the target area within the patient.

[0051] More specifically the catheter is loaded onto the proximal end ofthe guide wire 18 by the clinician who pushes the inner body 16 over theproximal end of the guide wire and then feeds the catheter down theguide wire. As this feeding operation occurs the guide wire 18 in effectpasses outside the catheter at the guide wire exit indicated at 20, inFIG. 3, the guide wire exit 20 being formed by the end of the inner body16.

[0052] The proximal end of the outer body 17 is sealingly secured to anouter tube 1 a of the catheter which contains an inner tube 1 b,typically a stainless steel hypodermic, the tubes 1 a and 1 b runningthe length of the catheter 1 up to a Y-connector 101.

[0053] A tapered metal member or wire known as a stilett 21 extends fromthe distal end of the inner tube 1 b into the space between the innerand outer bodies 16 and as illustrated in FIG. 3. The purpose of thestilett 21 is to provide a support for the guide wire exit port whichwould otherwise have a tendency to kink.

FIG. 4

[0054] This figure comprises FIGS. 4A, 4B, 4C, 4D and 4E which arecross-sections taken on the lines G3, G2, G1, respectively in FIG. 3,and F and E respectively in FIG. 1.

[0055] As can be seen from these cross-sections the ribbon cable 22consists of a number of electrical leads which for most of its lengthare moulded together to form the unit shown in FIGS. 4A, 4B and 4C.However, at the point where it is required to connect electrically thevarious constituents of the ribbon cable to the four multiplexer units 5the ribbon cable 22 is split into discreet leads as shown in FIG. 4D.These leads are then connected to the respective multiplexer units viatracks on the flex circuit.

[0056] The manner in which each multiplexer unit is mounted is shown inFIG. 4E. Each multiplexer unit is secured to the inner body 16 by meansof adhesive 23.

FIG. 5

[0057] This shows on a greater scale and in more detail the extremedistal end of the catheter, the same reference numerals being used todesignate parts already described with reference to earlier figures.

[0058] The annular ultrasonic transducer array 3 is contained between aproximal ceramic ring 24 and a stainless steel or ceramic distal ring25. At the distal end of the distal forming ring 25 there is the softtip 4, which is typically made from Nylon (RTM) which is heat melted orfused onto the distal end of the inner body 16.

FIG. 6

[0059]FIG. 6A shows that part of the catheter which includes thetransducer array 3 and multiplexers 5 when they are in the flatcondition at an intermediate point in the manufacturing process. A moredetailed disclosure of the method of manufacturing this arrangement,which involves first constructing the transducer and multiplexerassembly in the flat and then converting it into a tubular configurationis disclosed in more detail in our United Kingdom Patent No. 2 297 375

[0060]FIG. 6B shows the same arrangement as FIG. 6A but after it hasbeen formed into the cylindrical configuration referred to earlier.

FIG. 7

[0061] The present invention also envisages a stent, such as that shownat 102 in FIG. 1, being loaded with one or more of a variety of drugs sothat the drug or drugs is/are eluted or washed from the stent by thepatient's blood flowing past the stent.

[0062] By having a drug loaded stent mounted on an intravascularultrasonic imaging catheter (IVUS) it is possible for the clinician tomore accurately position the stent and target the drug where it isrequired in order to prevent, for example, restenosis. The usual methodof introducing a drug or drugs has been to simply introduce themgenerally into the patient's blood stream. However, this means that alarge proportion of the introduced drug is in effect wasted and is notoperative in the target area.

[0063] The loading of the drug or drugs onto the stent can be achievedin a number of ways.

[0064] A drug loaded surface of a stent can be achieved by usingdifferent technological approaches. Each of these approaches can beconducted in a way that the drug compound is released from the surfaceeither in a short (hours) or an extended time frame (days). The releasekinetics can be adjusted by applying specific modifications to thesurface of the stent e.g. hydrophobic or hydrophilic side chains of apolymer carrier or a ceramic surface.

[0065] The following outlines four possible ways of loading thedrug/drugs onto the stent.

[0066] 1. Ceramic Coating

[0067] An AlO₂ coating (patent applications DE 19855421, DE 19910188, WO00/25841) with porous surface can be loaded with a drug in amountsbetween 250 μg an 10 mg by either dipping, spraying or similartechniques. The drug dose is dependent on the nature of the targetvessel and the condition of the patient and is chosen such that cellproliferation is sufficiently inhibited, while healing is not hampered.The drug can be used as an aequous or organic solution, e.g. in DMSO,DMF and methanol. After spraying or dipping (under mild vacuum) thetreated stent is dried, the procedure is repeated three to ten times.After the final drying the stent is rinsed for one minute in water orisotonic saline at room temperature and then dried again. Drug contentcan be analysed by standard methods (HPLC, LC-MS) after eluting orwashing the compound with a suitable solvent. Release kinetics can beanalysed using a standard release apparatus.

[0068] 2. ePTFE Membrane: Stent Graft

[0069] Identical approach as above. The drug is absorbed into thecavities of the porous ePTFE membrane.

[0070] 3. Polymeric Coating in General

[0071] Different polymers are suitable for drug loading:methacrylate-polymers, polyurethane-coatings, ePTFE-coatings. The drugcan be either applied to the final surface (see above) or directly addedto the polymerisation solution.

[0072] 4. Mechanical Approach (FIG. 7)

[0073] The mechanical approach is based on holes 701 that have beenintroduced into the stent 700 struts 702 using a laser. These holes canthen be filled with a drug or drugs. The hole-approach can be combinedwith a thin, biodegradable coating that in itself is drug loaded. Afterinitial release from the biodegradable coating the drug-filled holes canserve for long term release of active drug. Interstices for containingthe drug may be formed in other ways than by holes.

[0074] A variety of drugs that could be loaded onto the stent are listedin the following three tables, Table A, Table B and Table C. It isintended that the listed drugs should also include their derivatives.

[0075] In this example the drugs are selected to be active in threephases, Table A being Phase I, Table B being Phase II and Table C beingPhase III.

[0076] Phase I is aimed at effecting vasodilation e.g. the dilation ofthe patient's artery. The effects are listed in the table.

[0077] Phase II is aimed at inhibiting inflammation, etc as listed atthe top of Table B. Again the effects of the drugs are set out in thetable.

[0078] Phase III is aimed at the inhibition of cell proliferation andagain the effects of the drugs are set out in the table. TABLE A PhaseI—Vasodilation Drugs to be released with the first 24-72 h afterstenting Drug name Rationale/Effects Priority Molsidomine, linsidomine,Release of NO leads to 1 sodium nitroprusside, vasodilation, reducingthe nitroglycerol, NO-donors in degree of procedural vessel general walldamage, stimulates the growth of endothelial cells, inhibits migrationand proliferation of smooth muscle cells. Stimulators of the soluble SGCstimulators induce 1 guanylate cyclase like BAY 41- vasodilation andother 2272 (5-(Cyclopropyl-2-[1-(2- NO-effects by directlyfluoro-benzyl)-1H-pyrazolo[3,4- activating the targetn]pyridin-3-yl]-pyrimidin-4- enzyme of NO. ylamine). Hydralazine. Causessmooth muscle cell 2 relaxation. Verapamil, dilitazem, nifedipine,Smooth muscle cell 3 nimodipine and other Ca²⁺ - contraction reduced bychannel blockers. reducing intracellular Ca²⁺ - concentrations.Captopril, enalapril, lisinopril, Reduction of the angiotensin 1quinapril and other angiotensin II level leads to reduced convertingenzyme inhibitors. vasoconstriction. Losartan, candesartan, Reducedvasoconstriction is 1 irbesartan, valsartan and other achieved byblocking the angiotensin II receptor effect of angiotensin II atantagonists. its target receptor located in the vascular tissue.

[0079] TABLE B Phase II—Inhibition of inflammation, immunosuppression,promotion of endothelial cell growth, inhibition of cell migration.Drugs to be released within the first 2-21 days after stenting. Drugname Rationale/Effects Priority Dexamethasone, Inhibition ofinflammatory reactions by 1 Betamethasone, different effects onmacrophages and prednisone and other monocytes, endothelial cells,basophils, corticosteriods fibroblasts and lymphocytes.17-beta-estradiol Inhibition of migration and proliferation of smoothmuscle cells. FK506 (Tacrolimus) Inhibition of T-cell response,reduction 1 of pro-inflammatory cytokine release, inhibition of smoothmuscle cell proliferation Cyclosporine Inhibition of T-cell response 3Mycophenolic acid Inhibition of B-cell response, inhibition 3 of smoothmuscle cell proliferation VEGF, VEGF-receptor VEGF is a growth factorstimulating the 1 activators growth of smooth muscle cells Tranilast Hasshown efficacy (prevention of 2 restenosis) in animal trials aftersystemic applications, inhibits keliod scarring. Mefoxicam, cefebrex,Antiinflammatory effect through 2 vioxx and other COS-2 inhibition ofcyclooxygenase 2 antagonists Indomathacin, Antiinflammatory effectthrough 3 diclofenac, ibuprofen, inhibition of cyclooxygenase 1, innaproxen and other addition platelet inhibition COX-1 inhibitorsPlasminogen activator Inhibits activation of prourokinase. 1 inhibitor-1and other Prourokinase promotes cellular serpins migration byacctivating plasmin and metalloproteinases as well as other proteinasesThrombin inhibitors as Thrombin promotes thrombus formation 2 hirudin,hirulog, but is also a strong mitogen agratroban, (growth factor) PPACKetc. Interleukin-10 Antiinflammatory cytokine that inhibits 3 monocytes

[0080] TABLE C Phase III—Inhibition of cell proliferation Drugs to bereleased within the first 14 days to 3 months after stenting Drug nameRationale/Effects Priority Sirolimus, SDZ Inhibition of T-cell response,reduction of 1 RAD (40-O-(2- pro-inflammatory cytokine release,inhibition hydroxyethyl)- of smooth muscle cell proliferation rapamycinand other rapamycin drivatives PDGF-antoginists Inhibition of smoothmuscle cell proliferation 1 and migration through inhibition of PDGF-signal transduction. PDGF is a strong mitogen for smooth muscle cells.Paclitaxel Inhibition of smooth muscle cell proliferation 1 throughpromotion of microtubili association Cisplatin Inhibition of smoothmuscle cell proliferation 2 through intercalation in DNA-double strandVinblastin Inhibition of smooth muscle cell 2 proliferation throughinhibition of mitotic spindle formation Mitoxantrone Inhibition ofsmooth muscle cell proliferation through inhibition of DNA and RNAsynthesis and inhibition of topisomerase II 1 Combretastatin Inhibitionof smooth muscle cell 1 A4 proliferation through inhibition of mitoticspindle formation Topotecan Inhibition of smooth muscle cell 2proliferation through inhibition of topoisomerase I MethotrexateInhibition of smooth muscle cell 3 proliferation through inhibition ofdihydrofolate reductase Flavopiridol Inhibition of smooth muscle cell 1proliferation through inhibition of cell cycle kinase

1. Apparatus for ultrasonic imaging, the apparatus comprising: acatheter having a distal region; an ultrasonic transducer array disposedat the distal region; and a flexible circuit electrically coupled to thetransducer array for supplying electrical power to the array, wherein atleast a portion of the flexible circuit is helically wound about thecatheter.
 2. The apparatus of claim 1, wherein the portion of theflexible circuit that is helically wound about the catheter is adaptedto enhance flexibility of the circuit as the catheter is moved along anon-linear path.
 3. The apparatus of claim 1, wherein the portion of theflexible circuit that is helically wound about the catheter is adaptedto accommodate bending the circuit as the catheter is moved along anon-linear path.
 4. The apparatus of claim 1, wherein the flexiblecircuit is insulated.
 5. The apparatus of claim 1, wherein the apparatushas a maximum external diameter of less than about 3 Fr.
 6. Theapparatus of claim 1, wherein the apparatus is adapted for disposalwithin guiding catheters as small as 6 Fr.
 7. The apparatus of claim 1,further comprising an imaging system coupled to the catheter.
 8. Theapparatus of claim 1, further comprising a plurality of multiplexerunits electrically coupled to the flexible circuit proximal of thetransducer array.
 9. The apparatus of claim 8, wherein the helicallywound portion of the flexible circuit is helically wound about thecatheter between the ultrasonic transducer array and the multiplexerunits.
 10. The apparatus of claim 8, wherein the multiplexer units arelongitudinally spaced with respect to one another along the length ofthe catheter.
 11. The apparatus of claim 10, wherein the flexiblecircuit is helically wound about the catheter between the multiplexerunits.
 12. The apparatus of claim 1, further comprises an expandableballoon coupled to the catheter.
 13. The apparatus of claim 12, whereinthe balloon is coupled to the catheter proximal of the transducer array.14. The apparatus of claim 13, further comprising a plurality ofmultiplexer units electrically coupled to the flexible circuit proximalof the balloon.
 15. The apparatus of claim 14, wherein the helicallywound portion of the flexible circuit is wound about the catheterbetween the ultrasonic transducer array and the multiplexer units. 16.The apparatus of claim 12, further comprising a stent coaxially disposedabout the balloon.
 17. The apparatus of claim 16, wherein the stent iscrimped onto the balloon.
 18. The apparatus of claim 17, wherein thehelically wound portion of the flexible circuit is adapted to limit aminimum diameter to which the stent may be crimped.
 19. The apparatus ofclaim 12, wherein a portion of the flexible circuit is disposed withinthe balloon.
 20. The apparatus of claim 19, wherein the helically woundportion of the flexible circuit is disposed within the balloon.
 21. Theapparatus of claim 19, wherein the portion-of the flexible circuitdisposed within the balloon is adapted for exposure to inflation mediumintroduced into the balloon.
 22. The apparatus of claim 19, wherein theportion of the flexible circuit disposed within the balloon isinsulated.
 23. A catheter which includes a balloon and a stent, thestent being pre-mounted on the balloon and the stent carrying a drugwhich is adapted to be eluted or washed from a stent into a patient'sblood stream when the stent has been deposited at a target locationwithin the patient.
 24. A catheter as claimed in claim 23 in which thedrug is applied to the stent as a coating.
 25. A catheter as claimed inclaim 23 in which the drug is lodged in interstices formed in the stent.26. A catheter as claimed in any one of claims 23 to 25 in which thestent carries a drug which has characteristics to effect vasodilation.27. A catheter as claimed in any one of claims 23 to 26 in which thestent carries a second drug to effect inhibition of inflammation,immunosuppression, promotion of endothelial cell growth and inhibitionof cell migration.
 28. A catheter as claimed in any one of claims 23 to27 in which a stent carries a third drug to effect the inhibition ofcell proliferation.
 29. A method of manufacturing a catheter as claimedin any previous claim in which the transducer array and multiplexerarrangement are first manufactured in a flat configuration and thenformed into a cylindrical configuration.